In increasing production of crude oil, a secondary/tertiary recovery process is used for the oil layer from which crude oil could not be collected according to a primary recovery process. The secondary/tertiary recovery process includes polymer flooding, micellar polymer flooding, etc. These flooding techniques are based on a principle of injecting a special aqueous polymer solution into an oily layer to extrude the residual oil, and further development thereof is strongly desired for increasing production of crude oil. The polymer to be used in these flooding techniques includes synthetic polymers such as acrylamide, etc.; natural polysaccharides and their derivatives such as xanthan gum, carboxymethyl cellulose and others (for example, Patent Literatures 1 to 4).